At one point there was a website for GM's that would decode up to 5 VIN's for you (before you had to start paying). I know I looked up a few GM trucks as I wanted to know what rear-end gearing they had. No idea if they are still around, that was a few years ago. [Some ads would list the VIN, but of course, no pic of the build sheet, so I couldn't get a list of the RPO's. The VIN decoder was able to tell me what gears were installed.]